The commercial extraction of oil as bitumen from oil sands involves the use of the "hot water" process in which mined oil sands typically are introduced into a rotating drum and slurried with steam and hot water at approximately 80.degree. C. The drum discharge, freed from rocks and clay lumps by screening, is further diluted with hot water to about 50% solids and a temperature of about 70.degree. to 75.degree. C., and pumped into a process vessel for the initial separation of bitumen from the oil sand slurry and recovery of bitumen as a primary froth product. The slurry discharged from the bottom of this vessel, and the middlings from an intermediate zone, are either further processed separately or combined and then processed by air flotation to recover additional bitumen from these streams. The flotation of bitumen in one or more vessels is termed a secondary recovery process. A sand-water slurry discharged from the bottom of these vessels becomes tailings and is discarded.
In secondary recovery processes, air is introduced into the slurry and the subsequent flotation of the bitumen yields a lower grade froth which contains higher contents of water and solids than obtained from the initial, or primary separation. The secondary froths are then combined into a settling vessel or "cleaner" where some of the excess water and solids are removed. Secondary froth is combined with primary froth to become the overall bituminous froth product. The cleaner bottoms slurry is returned to the flotation circuit.
The term "solids" used herein refers to inorganic solids such as fine quartz sand and silt and clay minerals.
In the commercial processes, bituminous froths produced in the secondary recovery circuit contain significant amounts of residual water and solids, e.g. 60 to 80% water and 5 to 10% solids. At the process temperature, solids and water partially separate from the bitumen resulting in a secondary bituminous froth containing approximately 30-35% bitumen, 50-55% water and 10-20% solids. This froth is combined with the primary froth which contains approximately 65% bitumen, 25% water and 10% solids. Combining the secondary froth stream with the primary stream results in the overall bituminous froth product.
In subsequent treatment of this froth, water and solids are further removed by dilution of the froth with a diluent solvent such as naphtha. This diluted bitumen is treated by centrifugation in the commercial process to remove water and solids. A reduction in the water and solids content of the froth would result in higher capacities in the centrifugation process and reduction in the hydrocarbon losses to the slurry.
Canadian Patent No. 857,306 issued on Dec. 1, 1970 to Dobson discloses the treatment of middlings by flotation to produce an aerated scavenger froth which is passed to a settling zone for separation of mineral matter from the froth. The separation occurs at the ambient temperature of the froth, normally 70.degree.-75.degree. C.
U.S. Pat. No. 3,338,814 issued on Aug. 29, 1967 to Given et al. describes a process whereby froths produced by hot water extraction of bitumen are dehydrated by heating to temperatures from 225.degree. to 550.degree. F. (preferably 350.degree. to 450.degree. F.). The dehydrated bitumen, containing 5% to 25% solids is then subjected to cycloning or filtration to remove solids. In a variation to the basic process, a light hydrocarbon can be added to the dry bitumen to improve the filtration step. The hydrocarbon can be recovered by distillation and recycled. This is essentially a two-stage process that requires a considerable amount of energy in order to obtain a satisfactory degree of water and solids removal.
U.S. Pat. No. 3,901,791 issued on Aug. 25, 1975 to Baillie discloses a method for upgrading bituminous froth by diluting the froth with a hydrocarbon diluent boiling in the range of 350.degree. to 750.degree. F., heating the diluted froth to a temperature in the range of 300.degree.-1000.degree. F. and settling the froth in an autoclave at a pressure in the range of 0 to 1000 psig, diluting settled tailings with the diluent and centrifuging the diluted tailings to provide a centrifugal froth.
U.S. Pat. No. 4,035,282 issued on Jul. 12, 1977 to Stuckberry et al. discloses a process for recovery of bitumen from a bituminous froth in which the froth is diluted with a hydrocarbon solvent and subjected to a two-stage centrifugation for removal of water and minerals. Solvent is added before each stage of centrifugation.
U.S. Pat. No. 4,648,964 issued on Mar. 10, 1987 to Leto et al. discloses a process for separating the hydrocarbon fraction from a tar sands froth in which the froth is pressurized to about 1000 psig and heated to about 300.degree. C. to enhance gravity separation, and the constituents separated at a reduced pressure.
U.S. Pat. No. 4,859,317 issued on Aug. 22, 1989 to Shelfantook et al. proposes three stages of inclined plate settlers to remove water and solids from bitumen froths. This process is carried out at approximately 80.degree. C. using naphtha as diluent in a 1:1 volume ratio based on the oil content in the froth.
Canadian Patent 915,608 issued on Nov. 28, 1972 to Clark et al. describes a process for removing water from a bituminous froth by imparting shearing energy to thereby coalesce water from at least 25 pounds of water per 100 pounds of bitumen to less than about 15 pounds of water per 100 pounds of bitumen. The process was carried out at temperatures between about 35.degree. to 49.degree. C.
The processes disclosed in the foregoing patents are complex and necessitate the use of expensive solvents or require high temperatures and/or pressures in an effort to beneficiate the bitumen froth.
It is the principal object of the present invention to provide a simple process and an apparatus for reducing water and inorganic solids from bituminous froths without the use of solvents.
Commercial extraction processes use water heated to a nominal temperature of about 70.degree. to 75.degree. C. Recent development work is aimed at reducing this processing temperature as low as 10.degree. C. to achieve energy savings and reductions in processing costs. However, reductions in processing temperature have the undesirable consequence of increasing the solids content in the froth products, thereby placing more emphasis on the development of froth cleaning processes to improve froth quality. In addition, froths produced at these low temperatures are extremely viscous and difficult to process.
It is another object of the present invention to provide a process and an apparatus to enable the production of high grade froth products from lower temperature oil sands extraction processes.